Switched-capacitor circuits are widely used for signal processing because of their low distortion and simple integration. Filters of many types may be realized with switched-capacitor circuits. In battery operated integrated circuits, for example, in general when operating at low voltage supply and at low power consumption, there exists the need for the circuits to function at particularly low voltages, such as down to about 1.5 V. Under these conditions it is difficult to efficiently drive the switches, which are commonly provided by field effect (FET) transistors, usually MOSFETs. Indeed, when the power supply is lowered down to levels comparable to those of the threshold voltage of the devices, the classical switched-capacitor structures as shown in FIG. 1, for example, may lose their efficiency rapidly.
To ensure correct functioning of the input switch S1, whose overdrive voltage depends upon the input signal, the operating voltage swing of the circuit should be limited. One proposed solution to this problem that would ensure that the switches, and in particular the input switch S1 see a high conductance under any input signal condition, is based on realizing the switches with low voltage threshold devices. Alternatively, dedicated clock voltage multiplier circuits could be integrated in the device (voltage boosters), with which the switches may then be overdriven. This second approach, while overcoming the problem of having to diversify the manufacturing process to realize low threshold devices, requires the integration of dedicated voltage boosters.
An alternative approach is based on the use of a switching structure called the "Switched Op-Amp" represented by an operational amplifier, as is disclosed in the article: "Switched-Opamp, an approach to Realize Full CMOS Switched-Capacitor Circuits at Very Low Power Supply Voltages" by Jan Crols and Michel Steyaert, Journal of Solid State Circuits, Vol. 25, No. 8, August 1994, pages 936-942. According to this approach, the proposed solution for ensuring a high conductance to the switches under any signal condition, and in particular to the input switch S1, rests upon substituting the MOSFET that is conventionally used as the input switch with a switched op-amp driven ON and OFF through a dedicated switch. The other switches of the switched-capacitor structure may be also realized with single n-channel or p-channel transistors, without necessarily resorting to CMOS structures. FIG. 7 of the above cited article shows the complete topology of a switched op-amp used as a switch to realize a switched-capacitor circuit. The circuit of FIG. 7 of the above mentioned article is herein reproduced as FIG. 2.
The switched op-amp circuit substantially includes two amplifying stages with a compensation capacitor CM according to a classical Miller compensation scheme. Unfortunately, this known amplifier presents a problem concerning the switching time. Being that the circuit is primarily destined for switched-capacitor circuits, where switched op-amps replace those simple switches that may provide critical elements in low voltage supply applications, the output node of the op-amp is driven to the supply voltage during a switching phase to thereby avoid the use of a critical switch along the signal path. Of course, its switching speed represents a fundamental limiting performance parameter.
In the illustrated circuit of FIG. 2, the switching time is strongly limited by the Miller capacitor CM. In fact, during a first switching phase Ph1 when the switched op-amp is OFF, the output node Vop is coupled to a very low reference voltage (or to ground) and therefore so is the plate of the compensation capacitor CM. The potential of the other plate of the capacitor CM also shifts because the first stage of the amplifier is OFF. This situation is reflected in an increased switch-on time because of the charge recovering time on the compensation capacitor CM.